(1) Field of the Invention
This invention relates generally to a device for measuring liquid level or volume of a contained fluid.
(2) Description of the Prior Art
The prior art of liquid level sensing can be divided into two categories: discrete sensing devices that sense whether a liquid level is above or below a certain point, or distributed sensing devices which transmit varying amounts of light depending upon the liquid level.
Scifres U.S. Pat. No. 4,287,427, filed Jan. 2, 1980 is an example of the latter. In this reference the attenuation of a light guide is changed depending upon the surface area of the light guide contacted by the liquid being measured. The principle used is the difference in the index of refraction between air and the liquid being measured. This approach inherently requires contact with the liquid being measured and cannot be used in case where contact is not allowed. This approach engenders liquid droplets which cling onto the sensor above the actual liquid level and distort the reading. These droplets may be formed from the liquid level being decreased from one level to a lower level.
Other types of sensing devices are essentially on/off sensors and do not allow measurement of liquid level as a continuous quantity (Hansel et al. U.S. Pat. No. 4,069,838, filed Jan. 24, 1978 is an example). These approaches use the local attenuation of light between a source and a detector to tell when the liquid has reached a particular level, or a change in the path of the light beam due to the liquid level which is sensed by the detector as a change in detected light. The Hansel et al. reference uses the attenuation approach. This method can only tell when the liquid level is above a certain point. It cannot give a continuous measurement of the liquid level. Furthermore, it requires contact with the measured liquid. Not all devices of this type require contact, such as Shapiro U.S. Pat. No. 3,741,656, filed June 26, 1973. All methods do suffer from the fact that they cannot give a continuous measurement over the full liquid level range. This is addressed by reference Kind U.S. Pat. No. 3,713,338, filed Nov. 30, 1973 by using multiple single level sensing devices to obtain a somewhat continuous measurement of the liquid level. This approach does require multiple sensors and has limited accuracy dependent upon the number of discrete sensing devices used. Additionally, the Kind reference requires contact with the measured liquid, although this method could be implemented without contact. This approach is one requiring complex hardware.
All prior art located either provides a non-contact measurement or a continuously variable measurement, but not both at the same time. In addition, no prior art reference is known teaching a non-contact measurement device that has a high degree of accuracy over a broad range.